This re to a test system for semiconductor circuit chips and more particularly to a secondary electron measuring circuit for measuring secondary electrons emitted from part of a semiconductor circuit chip to which an electron beam is applied or irradiated.
For analysis of the operation of a semiconductor integrated circuit chip, electron beams are applied or irradiated to some node portions in the semiconductor integrated circuit chip, and the amount of secondary electrons emitted from the node portions are detected or measured. The application or irradiation of the electron beam is effected by, for example, using a scanning type electron microscope (SEM). During the analysis of the operation, the integrated circuit is controlled in accordance with test data, for example, so as to periodically change the potential at a desired node portion or measurement point in a predetermined variation mode. The electron beam is intermittently generated in synchronism with the operation of changing the potential in the same manner as a strobe light, and is applied to each measurement point as beam pulses. Each pulse is generated in a preset phase relationship with respect to the period of potential variation at the measurement point, and is therefore applied to the measurement point each time the potential at the measurement point is set at a preset level. The amount of secondary electrons is measured each time the beam pulse is applied to the measurement point and measurements thus obtained are averaged to enhance the S/N ratio. The average of the detected amount of secondary electrons can be obtained by adding all the measurements and dividing the total sum by the number of measurements. The potential at the node has a correlated relation with respect to the amount of secondary electrons and can be derived from the average of the detected amount of secondary electrons.
In the operation analysis, it is checked to see if the potential at the measurement point changes in accordance with a designed variation mode. For this purpose, the application of electron beams is delayed by a predetermined phase amount with respect to the period of variation in potential at the measurement point each time the amount of secondary electrons are measured by a number of times required for the averaging process. The potential variation at the measurement point can be detected as a variation in the average of the amount of secondary electrons detected n each phase. Therefore, if it is detected that the potential variations at all the measurement points respectively conform to the designed variation, the integrated circuit can be determined to be properly performing its designed functions, and if it is not, the integrated circuit will be determined to be defective.
FIG. 1 shows the prior art secondary electron measuring circuit. When beam pulses are applied to the integrated circuit chip as described above, secondary electrons are emitted as periodic pulses from the circuit chip. The emitted secondary electron pulses are detected by means of secondary electron detector 10 including a photo multiplier or the like. The output pulses of secondary electron detector 10 are amplified by amplifier 12, smoothed by CR integrating circuit 14 and then converted into a digital signal corresponding to the measurement of the amount of secondary electrons by means of analog/digital (A/D) converter 16. The measurement signals are supplied to and averaged by a computer (not shown), for example. The secondary electron measuring circuit has the following disadvantages. That is, when the integrated circuit is formed at a high integration density to form an LSI, complicated test data must be supplied to the LSI circuit to initialize the potential of the measurement point. This lengthens the period of potential variation at the measurement point. In this case, it is necessary to increase the interval between successive applications of beam pulses or successive measurements of the amount of secondary electrons in accordance with the period of potential variation at the measurement point. In this case, since charges stored in a capacitor of CR integration circuit 14 are discharged over a period of time due to a leakage current, an output voltage of CR integration circuit 14 will be greatly lowered when the interval between measurements for the amount of secondary electrons is set relatively long. This deteriorates the S/N ratio of the measurements even if the measurements of the amount of secondary electrons are averaged. Such a problem cannot be neglected when it is taken into consideration that the current trend has been to increasingly integrate integrated circuits.
Now, a secondary electron measuring circuit disclosed in Japanese Patent Application (Application No. 56-21498) filed by the same applicant is explained with reference to FIG. 2. When secondary electrons are periodically emitted in the form of pulses from the integrated circuit chip, each of the secondary electron pulses is detected by secondary electron detector 20 which in turn produces corresponding output pulses. The output pulses of secondary electron detector 20 are supplied to peak holding circuit 22 and each of the peak levels thereof is temporarily held in peak hold circuit 22 and then converted into a digital signal and generated as a measurement of the amount of secondary electrons by A/D converter 24. The measurement is supplied to a computer (not shown) for the averaging process in such a manner as described above. The computer is also used to control the application of beam pulses. In the secondary electron measuring circuit, peak holding circuit 22 holds the peak level of an input pulse for a period corresponding to the interval between successive applications of beam pulses. Owing to this, the amount of secondary electrons can be correctly measured or converted into a digital signal even when the interval between successive applications of beam pulses is set long.
However, if each emission of the secondary electrons is constantly influenced, the potential variation at the measurement point cannot be correctly detected even if the number of measurements of the amount of secondary electrons is increased. Each emission of the secondary electrons may be interfered in the same manner by an electric field created in an area including the measurement point. Such an electric field may inadvertently change the direction of the emission of secondary electrons. This is known as a local field effect. The local field effect makes it difficult to measure the amount of secondary electrons at various measurement points under the same conditions.